Torrefaction kinetics of almond and walnut shells

Authors: Chiou, Bor-Sen; Cao, Trung; Valenzuela-Medina, Diana; Bilbao-Sainz, Cristina; Avena-Bustillos, Roberto; Milczarek, Rebecca; Du, Wen-Xian; Glenn, Gregory - Greg; Orts, William

Submitted to: Journal of Thermal Analysis and Calorimetry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/18/2017
Publication Date: 9/27/2017
Citation: Chiou, B., Cao, T.K., Valenzuela-Medina, D., Bilbao-Sainz, C., Avena Bustillos, R.D., Milczarek, R.R., Du, W.N., Glenn, G.M., Orts, W.J. 2017. Torrefaction kinetics of almond and walnut shells. Journal of Thermal Analysis and Calorimetry. volume 9, Article 643306. https://doi.org/10.1007/s10973-017-6721-6.
DOI: https://doi.org/10.1007/s10973-017-6721-6

Interpretive Summary: Torrefaction of biomass involves heating the sample under inert atmosphere at temperatures between 200 - 300 degrees Celsius for one hour or less. This results in a more stable and energy dense material that can be used as a renewable energy source or as fillers in polymer composites. In this study, we examined the effects of different inorganic species, such as potassium and sodium, on the torrefaction kinetics of almond and walnut shells. The almond and walnut industries produce hundreds of thousands of tons of shells each year. Torrefaction of these shells would provide an alternative to their current uses as bedding for livestock and abrasives. We found that samples containing higher concentrations of inorganic species resulted in lower mass yields. These results can be used to design reactors for large scale torrefaction of the shells.

Technical Abstract: The effects of inorganic species on the torrefaction kinetics of almond and walnut shells between 240 degrees Celsius and 300 degrees Celsius were determined using thermogravimetric analysis. Raw shells were leached with water at 80 degrees Celsius for 2 hours to remove much of the inorganic species. During each isothermal kinetics run, the leached almond shells had higher mass yields than the raw shells, even at 240 degrees Celsius. In comparison, the raw and leached walnut shells had comparable mass yields at 240 degrees Celsius with the mass yields becoming different at higher temperatures. This was due to the higher concentration of 0.89wt% potassium in almond shells compared to 0.08wt% in walnut shells, since potassium had been shown to catalyze decomposition of biomass. The two consecutive parallel reactions kinetic model exhibited the best fit to experimental data. However, the one step reaction model gave the best predictions of mass yields for torrefaction of raw almond shells in a fixed-bed reactor.